WASHINGTON (AP) — The United States and its European allies stepped up their pressure on Russia to end its intervention in Ukraine by imposing the most comprehensive sanctions against Russian officials since the Cold War.

Acting in concert with Europe, the Obama administration on Monday froze the U.S. assets of seven Russian officials, including top advisers to President Vladimir Putin, for their support of Crimea's vote to secede from Ukraine, while similar sanctions were imposed on four Ukrainian officials for instigating Sunday's Crimean referendum.

Although the threat of sanctions has failed thus far to persuade Putin to drop support for Crimea's secession and potential entry into the Russian Federation — or to pull back from threatening military moves near Ukraine's south and east — President Barack Obama said failure to step back now would draw more severe consequences.

"If Russia continues to interfere in Ukraine, we stand ready to impose further sanctions," Obama declared at the White House shortly after the penalties were announced. He noted that Vice President Joe Biden would be traveling to Europe late Monday to reassure Eastern European leaders of America's commitment to them and that he himself would be going next week on a previously planned trip to make a similar point. Secretary of State John Kerry also is expected in Europe in the coming days.

"We'll continue to make clear to Russia that further provocations will achieve nothing except to further isolate Russia and diminish its place in the world," he said. "The international community will continue to stand together to oppose any violations of Ukrainian sovereignty and territorial integrity, and continued Russian military intervention in Ukraine will only deepen Russia's diplomatic isolation and exact a greater toll on the Russian economy."

But administration critics said Obama's actions were too little to convince Putin of anything.

"I think Vladimir Putin must be encouraged by the absolute timidity," said Republican Sen. John McCain, who had just returned from a weekend trip to Kiev, the Ukrainian capital.

"I don't know how it could have been weaker, besides doing nothing — seven people being sanctioned after naked aggression has taken place," he told MSNBC.

Shortly after Obama's comments, Putin recognized Crimea as a "sovereign and independent country."

Residents in Crimea, a strategic Black Sea peninsula, voted overwhelmingly Sunday in favor of the split, and Crimea's parliament declared the region an independent state on Monday. In Washington, Obama administration officials said there was evidence that some ballots for the referendum had arrived pre-marked in many cities and there were "massive anomalies" in the vote. The officials did not say what that evidence was. The officials spoke to reporters on a conference call on the condition they not be quoted by name

The United States, European Union and others say that splitting off Crimea from Ukraine violates the Ukrainian constitution and international law and has taken place under duress from the Russian military. Putin maintains that the vote was legal and consistent with the right of self-determination.

President Barack Obama speaks about Ukraine, Monday, March 17, 2014, in the James Brady Press Briefi …

The U.S. announcement of sanctions came shortly after the European Union announced travel bans and asset freezes on 21 people the EU has linked to the unrest in Crimea. U.S. officials say there is some overlap between the U.S. list and that of the Europeans, which wasn't immediately made public.

The sanctions freeze any assets the targeted individuals have under U.S. jurisdiction, make it illegal for Americans to do business with them and discourage international banks and financial institutions from having relationships with them, administration officials said. The officials, however, would or could not say if those targeted actually have assets in U.S. jurisdictions.

In fact, one of the Russians named openly mocked the sanctions.

"Comrade Obama, what should those who have neither accounts nor property abroad do? Have you not thought about it?" Deputy Prime Minister Dmitry Rogozin tweeted. "I think the decree of the President of the United States was written by some joker."

U.S. officials said they also are developing evidence against individuals in the Russian arms industry and others who they said have helped to destabilize Ukraine.

President Barack Obama speaks about Ukraine, Monday, March 17, 2014, in the James Brady Press Briefi …

Putin wasn't a target of this round of sanctions because heads of state are not usually among the first to be penalized, the officials said. But those sanctioned are very close to Putin, they said.

"Today's actions also serve as notice to Russia that unless it abides by its international obligations and returns its military forces to their original bases and respects Ukraine's sovereignty and territorial integrity, the United States is prepared to take additional steps to impose further political and economic costs," a White House statement said.